Is Fasting Really a Safe Way to Lose Weight?

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Think of a time when you wanted to lose weight. Naturally, you knew to eat less, but how much less depends on your metabolism, which determines the total calories you need throughout the day. Eating fewer calories than you expend typically leads to weight loss, but how you want to budget these calories gets very personal.

While most of us prefer a consistent calorie goal, some find it helpful to slash calories through intermittent fasting. Intermittent fasting is skipping meal(s) for a certain number of days with the purpose of creating a calorie deficit for weight loss. On the days you don’t fast you have the freedom of a higher calorie goal to eat with.

Let’s be clear: Intermittent fasting is not:

1) Going more than 24 hours without eating.
2) Regularly eating a very low calorie diet (think: less than 1,000 calories for women and 1,200 calories for men).

Both these scenarios can lead to nutritional deficiencies and other health complications and are not something you should pursue without the help of a health professional. Now that we have that straight, let’s jump in:

Defining Fasting for Weight Loss
Intermittent fasting allows you to shift the calorie-cutting phase of your weight loss to a couple of days per week as opposed to every day. It works because you don’t need to cut calories daily to lose weight. You can go under your goal one day and over the next but still lose weight if you eat fewer calories than you burn.

It’s important to note not all fasting diets are the same, and that something as simple as skipping a meal can count as fasting. Here are a few different ways people practice fasting that don’t include religious fasting:

  • Complete alternate-day fasting: Fast every other day. A fasting day means no calorie-containing food or beverage for up to 24 hours. An eating day allows you to eat as much as you want.
  • Modified fasting: Fast just a couple of days per week. For example, the popular 5:2 diet schedules 5 days of eating freely with 2 days where calorie intake is 20–25% of needs.
  • Time- restricted eating: Eat freely as you feel hungry throughout the day, but only within a specific window of time. For example, setting up a window of 12 hours and only eating within that time frame. Skipping breakfast and not eating after dinner counts as time-restricted eating.

Source: Table adapted from “Intermittent Fasting and Human Metabolic Health” from the Journal of the Academy of Nutrition and Dietetics.

Are Fasting Diets Better than Nonfasting Diets?
A 2011 review by Krista Varady, PhD, at the University of Illinois at Chicago examined 11 daily-calorie-restriction and five intermittent-fasting studies. As an FYI, “daily calorie restriction” is how we’re traditionally taught to cut calories: cutting a consistent but small amount of calories every day. Varady found that both methods of cutting calories were equally effective at helping healthy subjects lose weight and fat, but intermittent fasting appeared to offer an added benefit. Those who practiced intermittent fasting preserved more lean mass (90% weight lost as fat, 10% weight lost as muscle) than those who followed the more traditional method of daily calorie restriction (75% weight lost as fat, 25% weight lost as muscle). Because lean mass burns more calories than fat mass, it is beneficial to help you lose fat while preserving as much muscle as possible.

While these numbers may look impressive, there are a few drawbacks to this review:

  • Not enough studies exist. The review included just 16 studies—there just weren’t many studies addressing intermittent fasting. Because of this, there were disproportionately more studies for daily calorie restriction—11 daily calorie-restriction studies versus five intermittent-fasting studies.
  • Small sample sizes. Study sample sizes were small, ranging from 8–173 subjects, with most studies hovering at 20 subjects.
  • Difference in measurement techniques. Most daily-calorie-restriction studies use dual-energy X-ray absorptiometry (DXA) and magnetic resonance imaging (MRI) to measure fat mass while most intermittent-fasting studies use bioelectrical impedance. Because DXA and MRI are more accurate at assessing fat mass compared to bioelectrical impedance, it makes it difficult to conclusively say that daily calorie restriction preserves lean mass better than intermittent fasting.

The science isn’t perfect, but it confirms what we already know: weight loss happens when the calories eaten are less than calories burned. If you’re overweight or obese, both nonfasting and fasting weight loss can lead to similar health benefits that’ll make your doc proud! Just a 5–10% weight loss can help bring down pesky numbers on your blood test (think: cholesterol, blood pressure and blood sugar).

Is Fasting a Safe Choice for Me?
The answer really depends on how you approach it and how your own body responds. As mentioned before, going long periods of time without eating, or eating a very low calorie diet for extended periods of time, is not safe. Generally speaking, though, the human body is designed to deal with periods of fasting. If you’re a healthy adult with weight to lose, there’s little evidence to say that intermittent fasting isn’t safe.

However, fasting isn’t for everyone and you shouldn’t try it if you are pregnant, diabetic or healing from a traumatic event like surgery. Still iffy about whether fasting is a good choice for you? We suggest seeking guidance from a health professional before experimenting.

5 Tips for Fasting
If you want to experiment with fasting to switch up your weight-loss journey, here are a couple of tips:

  1. Log your calories. If you’re doing a modified fast, you may want to track calories on a fasting day to help you meet your target calorie goal. For all other fasts, it’s helpful to track calories on the days you eat freely. Doing so can help offset the likelihood you’ll overeat and thus cut into your overall calorie deficit.
  2. Take it as an opportunity to better understand hunger. Not surprisingly, fasting diets can bring unpleasant side effects, one of which is hunger. We’re all born with hunger cues to signal that our stomach is empty and we should eat. For most of us, this ability becomes blunted as we age especially in a plentiful society where food is not in short supply. We may even mistake thirst or cravings for hunger. Fasting can reacquaint you with what physiologic hunger feels like.
  3. Drink plenty of water. Depending on how long your fast is, your body may switch to fueling itself more from fat and protein instead of carbohydrates. To do so will require more water to keep your metabolic machinery running smoothly. Also, if you’re already feeling minor unpleasant side effects from hunger, you don’t want to add a dehydration headache to the list!
  4. Choose nutrient-dense foods. On eating days, choose nutrient-dense foods that pack plenty of important vitamins and minerals along with calories. Nutrient-dense foods include fruits, vegetables, whole grains, dairy, lean meats, seafood, nuts—you get the idea! These are foods you should choose even if you’re not fasting. Check with your doctor if you’re concerned about vitamin and mineral deficiencies or want to undergo an extended fast longer than 12 weeks.
  5. Know you can always change your mind. Tried fasting and found it’s not for you? Fasting isn’t for everyone, especially those who get hungry really easily. Just know that you can cut calories the old-fashioned way and still lose weight.


  1. Varady, K. A. 2011. “Intermittent Versus Daily Calorie Restriction: Which Diet Regimen Is More Effective for Weight Loss?” International Association for the Study of Obesity 12(7): e593-e601. Accessed August 30, 2015.
  2. Patterson, R. E., G. A. Laughlin, A. Z. LaCroix, and S. J. Hartman. 2012. “Intermittent Fasting and Human Metabolic Health.” Journal of the Academy of Nutrition and Dietetics 115(8): 1203-1212. doi: http://dx.doi.org/10.1016/j.jand.2015.02.018. Accessed August 30, 2015.
  3. Orenstein, B. W. 2014. “Intermittent Fasting: The Key to Long-Term Weight Loss?” Today’s Dietitian 26(12): 40. Accessed August 30, 2015.

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